The invention relates to an arrangement of the type defined in the preamble of claim 1 and to an FM receiver equipped with such an arrangement.
Arrangements of this type are known from DE-OS 3,147,493 and DE-OS 3,438,286, EP-A1-75,071 or FR-OS 8,121,986, all of which originate from the inventor of the present invention.
In presently employed FM radio receivers, a broadband IF filter is provided. The center frequency f.sub.IF of such a broadband IF filter is 10.7 MHz in most standards while the channel bandwidth is about 200 kHz (.+-.100 kHz symmetrically with respect to the center frequency f.sub.IF). The passband characteristic is selected in such a manner that its bandwidth corresponds approximately to the transmission bandwidth. Due to the wide bandwidth, a relatively large amount of interference and noise components of the antenna input signal are passed by the prior art IF filter, with the result that relatively high input field strengths are required to ensure a useful signal (modulation) worthy of being received.
To increase reception sensitivity, it is known from the above-mentioned references to use as an IF filter a filter which has a narrow bandwidth relative to the channel bandwidth. The center frequency of this narrow-band IF filter is retuned in dependence on the modulation of the received signal. At any arbitrary point in time, selection thus occurs exactly where the momentary IF happens to be located. This results in much improved suppression of interference and noise components. The bandwidth of this controlled IF filter, which is also referred to in the literature as an "in-channel-select" filter or, abbreviated, "ICS" filter, is about 20 kHz as a result of which, however, only the monophonic useful signal component (L+R component of the useful MPX signal) is passed.
For this reason, it is only possible to operate the ICS filter in parallel with a broadband IF filter in an FM stereo receiver and to effectively connect the ICS filter with the LF stage of the receiver if, in any case, only monophonic reproduction is possible due to a received signal which is too weak or has too much interference (for example, below 25 .mu.V antenna input voltage).